- The Moon provides a stark snapshot of the violence of
our early solar system. Its largest craters are evidence of asteroid impacts
that scarred the surface between 3.8 billion and 4 billion years ago.
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- Scientists have long assumed Earth experienced a similar
bombardment, since we're so close to the Moon. Asteroids, comets and smaller
meteorites roamed the inner solar system in what was a final flurry of
devastation that might have delayed or challenged the onset of life, the
thinking goes.
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- However, no hard evidence exists, because unlike the
Moon, our planet is constantly folding its surface material into itself.
Rock is heated and melted, then later spewed back out through volcanoes
or sometimes brought to the surface by the planet's shifting continents
and crust.
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- Earth bears no craters from that time, and its surface
reveals little material that has been dated to that era.
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- A new study appearing in the July 25 issue of the journal
Nature adds some credence to the idea that the Late Heavy Bombardment,
as the 200-million-year period is called, also peppered Earth with rocks
from space.
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- In sedimentary rock that had previously been dated to
3.7 billion years ago or older, researchers found an isotope of the element
tungsten in amounts that do not typically occur on Earth and so are thought
to be of extraterrestrial origin. Isotopes are particles with identical
chemical properties, but different masses.
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- The material comes from two regions, one the Isua Greenstone
Belt in Greenland and another in northern Labrador, Canada.
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- The researchers did not find actual meteorites or chunks
of asteroids, explained lead scientist say Ronny Schoenberg of the University
of Queensland in Australia. Instead, they studied material that had long
ago been mixed with Earth's crust to form so-called metamorphosed sediments.
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- "These sediments consist of bits and pieces of weathered
rocks which have been transported by rivers into an ocean, where they first
formed soft layers, which were then compacted by more overlying sediments
and later deeply buried back into Earth's crust by subduction," Schoenberg
said in an e-mail interview. High pressure and temperatures crystallized
the sediments, and later they were returned to the surface.
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- While the study provides the first direct evidence that
the bombardment affected Earth, the mixing of the material puts a limit
on the conclusiveness of the findings.
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- "These metamorphosed sediments do not contain identifiable
pieces of meteorites or asteroids," Schoenberg said, and so "the
late heavy bombardment could not be proved."
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- The whole question of the bombardment is important because
many researchers believe life would have found it difficult to get going
under such chaotic conditions. An important question for biologists is
whether life arose on Earth only after the bombardment, or whether it might
have begun earlier and either had its development throttled by the influx
of asteroids and comets or, possibly, been reset entirely.
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- In a separate study reported earlier this week, researchers
say terrestrial material should have been kicked up during the bombardment
and transported to the Moon. If so, earthly rocks and dust would be waiting
there to be discovered. Some scientists believe that the only way the whole
question can be settled is to return to the Moon and search for this material.
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